Two projects are described in this section, whose only connection is that they deal with selective concentration (uptake) or selective toxicity of xenobiotics in the lung. Naphthalene, administered once to mice produced a significant (30-70%) and prolonged (8-15 days) impairment of pulmonary monooxygenase without altering these activities in liver microsomes. Inhibition of pulmonary enzymes was accompanied by selective necrosis of pulmonary nonciliated bronchiolar (Clara) cells with no evidence of morphologic damage to other pulmonary cell types. No morphologic changes were observed in sections of livers from these animals. In an extension of earlier work, 14C-imipramine and its major metabolite desmethylimipramine (DMI) were found to be selectively concentrated in lung relative to other tissues. Tissue/plasma (T/P) ratios for rat lung exceeded other tissues by 5-10 fold. Lungs of female rats concentrated more total drug, and more metabolite (DMI) than male lungs.